In recent years, the demand for magnetic recording materials which enable a large amount of information to be recorded in a small area thereof have increased. Consequently, the need of improvement in the dispersibility of a ferromagnetic fine powder has increased. Poor dispersibility of a ferromagnetic fine powder is responsible for low storage stability of the magnetic coating composition containing it, and decreases the signal output and increases the noise in the magnetic recording material using such a magnetic coating composition.
An improvement in the dispersibility of a ferromagnetic fine powder makes it possible for the powder to be given a high density charge and thereby, a high level of output can be gained and further, recording of a large amount of information in a small area becomes feasible. In addition to the high density recording, the magnetic recording material can be used under severe conditions in a recording and reproducing apparatus. However, various troubles may arise. For example, the magnetic layer coated may be worn off by repeated use thereby contaminating a running passage of the tape, or cause a drop in output of reproduced signals, fluctuation of output, drop out, an increase in noise and the like. One of the causes of these undesirable phenomena is the insufficient binding power of a binder to a ferromagnetic fine powder.
Special attention has been given with respect to dispersing a ferromagnetic fine powder into a binder as homogeneously as possible and to sticking the dispersed ferromagnetic fine powder firmly to the binder the preparation of a magnetic coating composition to be employed in a magnetic recording material. Known methods for preventing aggregation of a ferromagnetic fine powder from occurring in the magnetic coating composition and for heightening dispersibility of the ferromagnetic fine powder therein include (1) a method of adding to a magnetic coating composition a low molecular weight compound, e.g., ether compounds, quaternary ammonium salts, phosphoric acid esters, fatty acids, oxy compounds or the metal salts thereof, etc. as disclosed in Japanese Patent Application (OPI) Nos. 58804/74, 18006/75, 7704/77, 129410/77, 15802/78, 15803/78, 32304/79, etc., and U.S. Pat. No. 4,153,754, (2) a method of adding to a magnetic coating composition a high molecular weight compound as disclosed in U.S. Pat. No. 3,810,840; Japanese Patent Publication Nos. 9779/65, 22063/72, 9841/78, etc., and U.S. Pat. Nos. 3,630,771, 3,634,137 and 3,840,400, and (3) a method of treating a ferromagnetic fine powder with a coupling agent capable of reacting with a ferromagnetic fine powder, such as a silane coupling agent, a titanium coupling agent and the like, as disclosed in Japanese Patent Publication Nos. 125803/78, 125539/80, etc., and U.S. Pat. No. 4,330,600.
However, the above-described method (1) is not desirable because in order to impart excellent dispersibility to a magnetic coating composition it is necessary to add a large amount of low molecular weight compound. Accordingly, the resulting magnetic recording material can not have high mechanical strength, or the low molecular weight compound oozes out on the surface of a magnetizable layer causing a blooming phenomenon, which decreases the value of the recording material as a comodity or contaminates the running course of the tapes. The above-described method (2) is also defective because the mechanical strength of the magnetic recording material is detrimentally influenced by the high molecular weight compound added. Further, the method does not produce a tape with excellent abrasion resistance and a low coefficient of friction. The above-described method (3) may not be useful in practice because the surface treatment with a coupling agent is sensitive to moisture present at the surface of the ferromagnetic fine powder or the water content of the organic solvent used. Accordingly, it is difficult to control of the water content to the extent necessary for obtaining excellent dispersibility.